Our immediate objective is to focus on the acyl-CoA carboxylases, including acetyl-CoA carboxylase, a key enzyme in the regulation of fatty acid biosynthesis, and propionyl-CoA, 3-methylcrotonyl-CoA, and geranyl-CoA carboxylases, enzymes involved in odd-chain and branched-chain fatty acid catabolism. Our aim is to undertake an intensive structure/function analysis of the catalytic and regulatory subsites of different acyl-CoA carboxylases from the same source in order to answer questions on: a) the organization of subsites in these complexes; b) the degree of sharing of common structural features between pairs of homologous carboxylases (e.g., acetyl-CoA carboxylase versus propionyl-CoA carboxylase) from the same organism; c) the nature of the regulatory site(s) in acetyl-CoA carboxylase and its interaction with the catalytic sites; and d) the mechanism of assembly of multienzyme complexes and the advantages of the fusion of catalytic sites into multifunctional polypeptide chains.